systemd-conf:wired.network: do not send DHCP release on service stop

Fix the problem with floating IP addresses after first reboot in Edgehog
image.

Even when a stable MAC is assigned to device via device-tree, an IP address
may change after the first reboot.
This is due to default implementation of built-in DHCP client provided by
systemd-network daemon - DHCP RELEASE packet is sent to the DHCP server
during reboot by this daemon. So, next time a new IP address is allocated
to the same DHCP client (device).

To change this behavior of the DHCP client, a
"KeepConfiguration=dhcp-on-stop" option is enabled in configuration file
for systemd-networkd.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>

recipes-core/systemd/systemd-conf/wired.network-do-not-send-DHCP-release.patch

+--- a/wired.network
++++ b/wired.network
+@@ -6,6 +6,7 @@
+ 
+ [Network]
+ DHCP=yes
++KeepConfiguration=dhcp-on-stop
+ 
+ [DHCP]
+ UseMTU=yes

recipes-core/systemd/systemd-conf_%.bbappend

+FILESEXTRAPATHS:prepend := "${THISDIR}/systemd-conf:"
+
+SRC_URI += " \
+    file://wired.network-do-not-send-DHCP-release.patch \
+"
+
+S = "${WORKDIR}"